Direct hydration of ethylene to ethanol

ABSTRACT

Ethanol is produced through the direct hydration of ethylene by reacting a mixture of ethylene and water in the vapor phase over a supported sesqui-phosphoric acid catalyst. The preferred catalyst species is the hemihydrate of meta phosphoric acid. Typically, the direct hydration reaction is conducted at elevated pressures and at temperatures above the dew point of water at reaction conditions.

United States Patent Britton 1 Aug. 22, 1972 DIRECT HYDRATION OFETHYLENE 3,340,313 9/ 1967 Mitsutani ..260/641 TO ETHANOL 3,459,6788/1969 Hagemeyer et a1 ..260/641 [72] Inventor: Minebrook OTHER [73]Assignee: Esso Research and Engineering Comp g gi" 8th edmon Merck &Rahway 22] Filed; Jam 13 19 9 Primary Examiner-Howard T. Mars [21] ApplNO 790 831 Att0mey-Chasan and Sinnock and J. E. Luecke [57] ABSTRACTEthanol is produced through the direct hydration of [58] Fie'ld 260/641ethylene by reacting a mixture of ethylene and water in the vapor phaseOver a Supported sesqui phosphoric 56 R f d acid catalyst. The preferredcatalyst species is the 1 e erences le hemihydrate of meta phosphoricacid. Typically, the UNITED STATES PATENTS direct hydration reaction isconducted at elevated 2 486 980 11/1949 Robinson 260/641 pressures andat temperatures above the dew point of 2 496 621 2 1950 Deer.....IIIIIIITIIIII260/641 water ream condimns' 2,569,092 9/1951 Deering..260/641 X N1SOH et a1 6 N0 Drawings 2,673,221 3/1954 Schrader et a1..260/64l 3,311,568 3/1967 .lakov Levich ..260/641 X DIRECT HYDRATION OFETHYLEN E TO ETHANOL BACKGROUND OF THE INVENTION 1 Field of theInvention This invention relates to an improved process for theformation of ethanol through the direct hydration of ethylene. Moreparticularly, this invention is directed to the preparation of ethanolby the reaction of a mixture of water and ethylene at elevatedtemperatures and pressures in the presence of a supportedsesquiphosphoric acid catalyst.

2. Description of the Prior Art There are numerous literaturereferencesthat teach the direct hydration of olefinic feedstocks toalkanols. For example, it is reported in British Pat. No. 586,282 thatolefins can be catalytically hydrated to the corresponding alkanolsusing sulfuric acid as a catalyst. Anionic ion-exchange resins have alsobeen employed as catalysts in the direct hydration of monoolefins toalcohols (see: US. Pat. No. 2,803,667). Supported tungstic acidcatalysts have also been employed in promoting the hydration of olefinsto alcohols (see: British Pat. Nos. 708,623 and 703,628). The use ofphosphoric acid materials to catalyze the direct hydration reaction isalso widely reported with ortho phosphoric acid being the preferredcatalyst species. While numerous direct hydration techniques have beenreported, the prior art operations are invariably characterized byextremely low olefin conversion. Accordingly, the alcohol yieldsnormally secured are relatively small.

SUMMARY OF THE INVENTION Now, in accordance with the present invention,it has been found that the yields normally associated with the directhydration of ethylene to ethanol can be improved significantly bycontacting a mixture of ethylene and water over a supportedsesqui-phosphoric acid catalyst at elevated temperatures and pressures.Desirably, the catalyst is the hemihydrate of meta phosphoric acid. Thehydration reaction is conveniently conducted in the vapor phase bypassing a gaseous mixture of ethylene and water over a fixed bed ofcatalyst at temperatures ranging from 500 to 600 F. at an elevatedpressure consistent with the prevention of the condensation of the waterreactant at the condi tions in the reaction zone.

It is preferred that the ethylene and water reagents employed in theprocess be substantially pure. Desirably, the ethylene fed to thereaction zone is at least about 99 wt. percent ethylene. Acetylenicimpurities should be particularly avoided in that their presence resultsin the formation of undesired byproduct materials. Process impuritiesnormally associated with ethylene and water do not materially affectcatalyst performance; however, their presence normally causes theprocess to be plagued with purge and recycle problems. It is highlydesirable, although not es sential, that the reaction zone be free ofany substantial uncontrolled quantities of liquid water because liquidwater serves to hydrolyze the phosphoric acid catalyst to an undesiredcatalyst specie and may also serve to degrade the catalyst supportstructure. The vaporized water and ethylene may be introduced into thereaction zone either as single streams or as a mixture. The molar ratioof water to ethylene within the reaction zone may vary from about 01:1to about 20:1; however, it is preferred that the water/ethylene molarratio within the reaction zone range from about 0.521 to 1.5:]. Mostpreferably, the molar ratio of water to ethylene within the reactionzone is maintained at about 0.65 moles of water per mole of ethylene.

The direct hydration reaction is conducted at temperatures ranging from500 to 600 F., preferably at temperatures varying from 520 to 540 F.Preferably, the reaction zone pressure is maintained at the highestlevel possible without causing condensation of the water vapor presentat the conditions prevailing within the reaction zone. Typically,reaction zone pressures vary from 800 to 1,100 psig, most preferablyabout l,000l,050 psig are employed. The vaporized ethylene and water areintroduced into the reactor, preferably a fixed bed reactor, at a vaporspace velocity of about 10 to 60 standard cubic feet of total ethyleneand water vapor per cubic foot of catalyst per minute where standard gasconditions are understood to be 1 atmosphere and 60 F. However, it ispreferred that the reaction zone space velocity be maintained at a levelvarying between 25 and 35 standard cubic feet of ethylene and watervapor per cubic foot of catalyst per minute.

The nature of the catalyst employed in promoting the instant directhydration reaction is a critical element of the process. As notedearlier, the catalyst employed consists of a sesqui-phosphoric acidactive ingredient impregnated on an inert support material. Sincephosphoric acid exists in many forms, various nomenclatures havedeveloped in identifying the material. One typical technique forcharacterizing phosphoric acid materials is the relative molar ratios ofphosphorus pentoxide (P 0 to water making up the (polymeric) acidstructure. Hence, with ortho-phosphoric acid, the molar ratio ofphosphorus pentoxide to water is 1:3. With pyrophosphoric acid, themolar ratio of phosphorus pentoxide to water is 1:2. Meta phosphoricacid is conventionally characterized as being composed of a l: 1 molarratio of phosphorus pentoxide to water.

The preferred active catalyst specie is a sesquiphosphoric acidcharacterized by a P O :H O mole ratio within the limits of l: l to l:2. The most preferred catalyst specie is the hemihydrate of metaphosphoric acid, that is, a material having a phosphorus pentoxide towater molar ratio of about 1:15. Hence, the preferred active catalystspecies exist between materials typically denominated as pyrophosphoricacid and metaphosphoric acid. While the catalyst initially charged tothe reaction zone may be of a type having a relative phosphoruspentoxide to water molar ratio falling outside the desired limits, it isnecessary that the actual operating catalyst be of a composition fallingwithin the above ranges during the time of reaction. Catalysts withexcessive amounts of water or with excessive amounts of phosphoruspentoxide may be brought within the desired limits by carefullycontrolling reaction temperature and/or water content of the incomingethylene/water reactant streams.

The active sesqui-phosphoric acid catalyst materials are preferablysupported on inert materials that are not seriously degraded at processconditions. Useful supports include materials of predominantly siliceouscharacter such as diatomaceous earth, kieselguhr, and

porous silica and alumina-silica materials. Other useful materialsinclude coal, asbestos, charcoal, silicon carbide, etc. Thesesqui-phosphoric acid normally makes up the major amount of the totalcatalyst material. Typically, the phosphorus content of the totalimpregnated catalyst, calculated as phosphorus pentoxide, is in theorder of from 50 to 80 wt. percent of the total catalyst material.

The supported catalyst structures are typically prepared by admixingspecified amounts of water and phosphorus pentoxide with a siliceousabsorbent material. This crude mixture is then calcined at a temperatureof 900 to l,lO F. to form the essentially complete catalyst structure.As noted earlier, the catalyst from the calcining operation may notcontain the desired P O zwater molar ratios; however, the catalystcontent must be at a level such that it can be adjusted within thereaction zone to the desired critical levels.

In a typical reaction procedure, a mixture of water and ethylene ispassed through a furnace and brought to reaction temperature. Thepreheated vaporized mixture is then introduced into a tubular reactorcontaining a fixed bed of extruded diatomaceous earth pelletsimpregnated with a sesqui-phosphoric acid material. The temperaturewithin the catalyst bed is maintained at about 520 to 540 F. andreaction zone pressures maintained at about 1,030 psig. The productsfrom the fixed bed reaction are then condensed at an appropriatetemperature thereby recovering the product ethanol and unreacted wateras well as by-product diethyl ether. The unreacted ethylene is admixedwith further amounts of water and returned to the reaction zone asrecycle material. The by-product diethyl ether after separation from theethanol/water stream can also be recycled to the reaction zone to beconverted to ethanol product.

The amount of ether condensed is largely determined by the choice ofcondensor temperature. It is desirable to condense and recycle as muchof the ether as possible since the selectivity of ethylene converted toethanol is thereby improved. However, if condensation of the reactionproducts takes place under pressure, care must be taken to keep thetemperature high enough to prevent solid ethylene hydrate formation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will be furtherunderstood by reference to the following Examples.

Example 1 To demonstrate the effectiveness of the instant process, aseries of tests were conducted wherein the sesqui-phosphoric acidcatalyst system of the invention and a conventional ortho phosphoricacid based catalyst system were employed for the direct hydration ofethylene to ethanol. Each of the experimental runs were conducted bypassing vaporized ethylene and water over a fixed bed of about 34 cc ofcatalyst particles having an average particle diameter of aboutonesixteenth inch. The particles were contained within a tubular reactorhaving an inside diameter of one-half inch. The sesqui-phosphoric acidcatalyst material was composed of a porous alumina-silica support havingan elemental composition of about 3.5 wt. percent A1 0 and 96.5 wt.percent SiO on which sesqui-phosphoric acid was impregnated. Thesesqui-phosphoric acid content calculated as P 0 was about 54.6 wt.percent with sufficient water to yield a mole ratio of P 0 to H O of1:1.5.

In each of the tests, reaction zone pressure was maintained at about1,030 psig and the reaction temperature was varied over a range of 500to 700 F. The incoming reactant stream was composed of about 0.65 molesof water per mole of ethylene. The vaporized stream was passed throughthe tubular reactor at a vapor space velocity of 33 standard cubic feetper cubic foot of catalyst per minute. The results of the tests are setforth in Table I below.

*The ortho phosphoric acid catalyst consisted of substantially the samesupport but the mole ratio of P 0 :H. O was 1:3.

The above data clearly indicate the superiority of the instantsesqui-phosphoric acid catalyst systems for promoting the directhydration of ethylene to ethanol. In contrast, when a conventional orthophosphoric acid based catalyst system was used the percent ethanol yieldwas substantially below that secured when the sesqui-phosphoric acidcatalyst system was used. In this regard a comparison of Run 3 with Run5 indicates that approximately a 50 percent increase in ethanol yield issecured at identical conditions using the sesquiphosphoric acid basedcatalyst system.

Having thus described the general nature and specific embodiments of thepresent invention, the true scope of the invention is now pointed out inthe appended claims.

What is claimed is:

l. A process for the production of ethanol which comprises contactingethylene and water in a water/ethylene mole ratio of about 0.65 moles ofwater per mole of ethylene within a reaction zone in the vapor phaseover a catalyst, said catalyst having a phosphorous content, measured asphosphorous pentoxide, of from 50 to wt. percent of the total weight ofthe catalyst and, consisting essentially of sesquiphosphoric acidimpregnated on a carrier, said sesquiconducted at a temperature rangingfrom 520 to 540 F. at a pressure of from about 1,000 1,050 psig.

4. The process of claim 3 wherein said sesquiphosphoric acid is thehemihydrate of meta phosphoric acid.

5. The process of claim 4 wherein said carrier material is a porousalumina-silica material.

6. The process of claim 2 wherein said siliceous absorbent material isdiatomaceous earth.

2. The process of claim 1 wherein said catalyst carrier is a siliceousabsorbent material.
 3. The process of claim 1 wherein said reaction isconducted at a temperature ranging from 520* to 540* F. at a pressure offrom about 1,000 - 1,050 psig.
 4. The process of claim 3 wherein saidsesqui-phosphoric acid is the hemihydrate of meta phosphoric acid. 5.The process of claim 4 wherein said carrier material is a porousalumina-silica material.
 6. The process of claim 2 wherein saidsiliceous absorbent material is diatomaceous earth.